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Neurophysiology (3)

  1. Neurophysiology (3) Abbas A. Abbas Shawka Medical student / 2nd grade
  2. Subjects •Neural circuit in cerebellum •Functions of cerebellum •Lesions of cerebellum
  3. Introduction • Cerebellum executes its functions through excitatory output of the deep cerebellar nuclei to the brain stem and thalamus. • Neural connections within the cerebellar cortex, i.e. intrinsic • cerebellar circuit is basically concerned with modulating or timing the excitatory output of deep cerebellar nuclei via the fibres of Purkinje cells. • This is done in accordance with the signals received by the cerebellar cortex from different parts of the brain and body. The entire process can be discussed, for the purpose of understanding only, under following headings: 1. Afferents to cerebellar cortex. 2. Neuronal activity of intrinsic cerebellar circuitry. 3. Neuronal activity of deep cerebellar nuclei.
  4. Afferents to cerebellar cortex • Afferents to cerebellar cortex reach via two types of fibres: • 1. Climbing fibres. These fibres represent terminations of axons reaching the cerebellum from the inferior olivary nucleus. The climbing fibres excite the Purkinje cells directly and the deep cerebellar nuclei via the collaterals by releasing the excitatory neurotransmitter aspartate. • 2. Mossy fibres. All the afferent fibres entering the cerebellum, other than the olivocerebellar, are called mossy fibres. These fibres after reaching the granular layer of cerebellar cortex branch profusely and then each branch terminate within a glomerulus. The glomerulus also receives axon terminals of Golgi cells ( they excite the purkinje cells through granular cells !! – indirectly - )
  5. Afferents to cerebellar cortex
  6. Neuronal activity of intrinsic cerebellar circuitry • As a result of excitatory input from the climbing fibres or mossy fibres following activity is set up in the intrinsic cerebellar circuitry. • Feed forward inhibition for purkinje cells • Granule cell ( + )  purkinje cell • Granule cell ( + )  basket cell ( - )  purkinje cell
  7. Neuronal activity of intrinsic cerebellar circuitry • Feed forward inhibition of granule cells • Granule cell ( + )  purkinje cell • Granule cell ( + )  Golgi cell ( - )  purkinje cell • Feedback inhibition of granule cells • Granule cell ( + )  Golgi cell ( - )  granule cell !!?
  8. Neuronal activity of intrinsic cerebellar circuitry • Reverberating circuit • Complex process • Involved in rapidly alternating movement ( for example when asked someone to pronate and supinate his arm rapidly !! )
  9. Neuronal activity of deep cerebellar nuclei • Deep cerebellar nuclei are excitatory !! • Received fibers from :- • 1- purkinje cells ( inhibitory / GABA ) • 2- mossy fibers • 3- climbing fibers • 4- others • Send fibers to thalamus and brain stem nuclei and that is how the cerebellum function in co-ordination !!
  10. Cerebellum inputs Area Fibers Function 1 Cortex Cortico-ponto-cerebellar Cortico-olivo cerebellar Cortical control 2 Posterior horn cells of spinal cord Anterior and posterior spinocerebellar tract Information from muscle and joints 3 Cuneate nucleus Cuneocerebellar Information from muscle and joints 4 Vestibular nuclei Vestibulocerebellar Information about head position and eye movement Cerebellum Spinal cordBrainstemCortex
  11. Cerebellum outputs Area Fibers Function 1 Contralateral red nucleus Cerebellorubral Ipsilateral motor activity 2 Thalamus Dentato-thalamic Ipsilateral motor activity 3 Vestibular nuclei Cerebellovestibular Ipsilateral extensor muscle tone 4 Reticular formation Cerebelloreticular Ipsilateral muscle tone Thalamus brainstem Cerebellum
  12. Superior cerebellar peduncle • Connect cerebellum to midbrain • Main output from cerebellum ( efferent ) • Small afferent !! • Efferent :- 1. Cerebello-rubral fibers 2. Dentato-thalamic fibers 3. Cerebello-vestibular fibers • Afferent fibers • Anterior spinocerebellar tract • Rubrocerebellar • tectocerebellar fibers ( auditory and visual information )
  13. middle cerebellar peduncle • Connect cerebellum to pons • Main input to cerebellum !! • Composed exclusively from cortico-ponto-cerebellar fibers.
  14. Inferior cerebellar peduncle • Connect the cerebellum to medulla oblongata .. • Efferent :- • Cerebello-olivary fibers • Cerebello reticular fibers • Afferent :- • Posterior spinocerebellar fibers • Cuneocerebellar fibers • Reticulocerebellar • Vestibulocerebellar • Olivocerebellar ( olive receive from cortex, spinal cord and brain stem )
  15. Sensory representation on cerebellum • Double representation ( on superior surface and on inferior surface ) … • Above : - ipsilateral inverted representation … - Axial in vermis and limbs and other structures in the paramedian region. • Below : - bilateral projection • Special auditory and visual area also can be indicated ..
  16. Functional division of cerebellum • Functionally, cerebellum is divided into three divisions: • 1. Vestibulocerebellum. - It includes the flocculonodular lobe plus part of vermis. - which is its principal component and has vestibular connections only. - Nucleus fastigial is its effector nucleus. - It is concerned with control of body posture and equilibrium.
  17. Functional division of cerebellum • 2. Spinocerebellum. - includes vermis and paravermis zone - Nucleus interpositus, i.e. nucleus globossus and nucleus emboliformis are its effector nuclei. - It receives proprioceptive inputs from the spinal cord and is concerned with control of axial (trunk) and limb muscles postural reflexes.
  18. Functional division of cerebellum • 3- Corticocerebellum. - Lateral part of the cerebellar hemisphere - Nucleus dentatus is its effector nucleus. - It occupies the more lateral regions of the cerebellar cortex and receives information from the cerebral cortex and pons. • It is concerned with smooth performance of highly skilled voluntary movements.
  19. Vestibulocerebllum ( Archycerebellum ) Part Vetibulocerebellum ( Archicerebellum ) Positon Flocculonodular lobe + part of the vermis Cerebellar Nucleus Fastigial n. Connection Vestibular n. Tract Vetibulospinal Function Balance in spatial orientation
  20. spinocerebllum ( palocerebellum ) Part spinocerebellum ( palocerebellum ) Positon Vermis + paravermis zone Cerebellar Nucleus Interposed n. ( globosus + emboliformis ) Connection • DC • Spinocerebellar system Tract Rubrospinal Reticulospinal Function Tone and fine tuning
  21. cerebrocerebllum ( neocerebellum ) Part cerebrocerebellum ( neocerebellum ) Positon Lateral part of hemisphere Cerebellar Nucleus Dentate n. Connection • Cerebral cortex • thalamus Tract Corticospinal Function - Planning movemen - Evaluation the sensory information for action - Cognitive and speech
  22. Functions of the cerebellum 1. Control of the body balance and equilibrium through the vetibulocerebellum. 2. Control of muscle tone and stretch reflex through the spinocerebellum. 3. Control of voluntary movement through cerbrocrebellum. 4. Others a. Affecting the ANS b. Affecting the ascending sensory pathway c. Affecting eyeball movement
  23. Control of balance and equilibrium • Afferent • Vetibulocerebellar • Spinocerebellar • Cuneocerebellar • Reticulocerebellar • Efferent • Cerebellovestibular fibers • Cerebelloreticular fibers • the cerebellum influence the spinal motor neurons to keep the body posture upright through the vestibulospinal and reticulospinal tracts, and regulate the position of eyes in relation to movements of the head by connecting motor nuclei of extraocular muscles (3rd, 4th and 6th cranial nerves) via medial longitudinal fasciculus.
  24. Muscle tone and stretch reflex • Afferent • Spinocerebellar • Cuneocerebellar • Olivocerebellar ( ?! ) ( It carries proprioceptive impulses from the whole body and output signals from the cerebral cortex ) • Efferent • Cerebello-olivo-spinal tract • Cerebello-reticulo-spinal tract • The above 2 efferent excite the gamma motor neuron • The gamma motor neurons reflexly modify the activity of alpha motor neurons and thus regulate the muscle tone. • Thus cerebellum forms an important site of linkage of α-γ systems responsible for muscle tone.
  25. Voluntary movement • Open feedback loop • Function ? • Modulates the motor command of pyramidal tract with a programming of movement.
  26. Voluntary movement • Closed feedback loop • Function ? •LATER
  27. How cerebellum control the voluntary movement ?! • Via 5 mechanisms all work together … • 1- comparator function • 2- damping function • 3- timing and programming the movement • 4- control of rapid alternating movement • 5- servomechanism
  28. 1. Comparator function a. When the motor cortex sends impulses through the corticospinal tracts to the lower motor neurons for commanding movements of exploratory nature, it sends messages on the way to the cerebellum about the sequential intended plan of movements for the next fraction of second. b. The cerebellum also gets feedback from the proprioceptive endings of muscles, tendons and joints about what actual movements result. c. Comparison ( thus it is comparator function !! ) d. cerebellar may comment on every command of motor cortex ( 10-20 mS ) e. Closed loop circuit ?! f. By its comparator action, the cerebellum provides smooth co-ordinate movements of agonist and antagonist muscles of the limbs for the performance of acute purposeful patterned movements
  29. 2. Damping function • Prevent overs • all the movements are pendular and have a tendency to overshoot. The corticocerebellum sends impulses to the cerebral cortex to discharge appropriate signals to the muscles so that, any extra or exaggeration of muscular activity does not occur and thus prevents the overshooting.hooting of action …
  30. 3. Timing and programing the movement • Planning of sequence movement - Communication between cerebellum and cerebrum provides appropriate transmission from one movement to another … - Learning process may overcome this step ?! • Timing function • Predicting events ( What will happened next … ! )
  31. 4. Control of rapid alternating movements • Like dancing • May be explained by the reverberating circuit .. • Ex. Rapid movement of arm in pronation and supination movements …
  32. 5. Servomechanism of cerebellum • Cerebellum teach cerebral cortex !! • Cerebral cortex can work alone NOW • If any disturbance occur, we DO NOT WORRY because there is CEREBELLUM !!! • This is servomechanism action of cerebellum
  33. Cerebellar lesions • Signs of cerebellar dysfunction !! 1- disturbance in tone and posture 2- disturbance in equilibrium 3- disturbance in movement
  34. 1. Disturbinace in tone and posture • Atonia ( reduced output of cerbrocerebellum ) • Attitude changes in the unilateral lesions of the cerebellum are: - Rotation of the face towards opposite side (pulled by the healthy muscles). - Lowering of the shoulder on the affected side. - Outward rotation and abduction of the leg on involved side. Trunk is bent with concavity towards the affected side; this is because the weight of the body is thrown on the unaffected leg. • Deviation movement. - The arm held straight out in front of the body, deviates laterally when the eyes are closed. - In bilateral lesions both arms deviate. • 4. Effect on deep reflexes. - The deep or tendon reflexes become weak and pendular. - For example, pendular knee jerk in which after the initial reflex response the leg shakes several to and fro movements before it comes to rest. It occurs due to hypotonia of the quadriceps muscle.
  35. 2. Disturbinace in equilibiium • walks on a wide base. • sways from side to side (drunken like gait) • unable to maintain the upright posture and falls on closing the eyes (Romberg’s sign) due to involvement of vestibular system.
  36. 3. Disturbinace in movment 1. Ataxia - lack of co-ordination of movements, is th hallmark of cerebellar disorder. - It is characterized by: a) Decomposition of movements, i.e. the movements see to occur in stages at different joints b) Asynergia, i.e. lack of co-ordination between the protagonist, synergist and antagonist muscles. c) Dysmetria, i.e. movements are incorrect in range direction and force. The movements may overshoot their intended mark (hypermetria) or fall short of it (hypometria).
  37. 3. Disturbinace in movment 2- intension tremor - become evident during purposeful movements, and diminish or disappear with rest. - observed at the end of movement - coarse, oscillating, to and fro and rhythmic. 3- Nystagmus - refers to the regular and rhythmic to and fro involuntary oscillatory movements of the eyes, occurring due to inco- ordination of extraocular muscles. - Cerebellar nystagmus occurs during damage to flocculonodular lobes and occurs at rest (when neither the person nor the visual scene is moving).
  38. 3. Disturbinace in movment 4- Slurred speech - occurs due to in coordination of various muscles and structures involved in speech. 5- Astasia - refers to unsteady voluntary movements. Charcot’s triad It is a syndrome characterized by nystagmus, intention tremors and scanning speech seen in disseminated sclerosis causing disturbance of connection of cerebellum with brain stem.
  39. Clinical test for cerebellar dysfunction 1. Finger–nose test :- Intension tremor more severe when we reach the face. 2. Adiadochokinesia, i.e. the patient is unable to rapidly perform alternating movements, e.g. supination and pronation, of the forearm. 3. Rebound phenomenon. When the patient attempts to do a movement against a resistance, and if the resistance is suddenly removed, the limb moves forcibly in the direction towards which the effect was made. This is called rebound phenomenon. It is due to the absence of breaking action of antagonistic muscles. 4. Gait test. When the patient is asked to walk on a straight line, he is unable to do so (even with eyes open); he follows a zigzag path. 5. 5. Past pointing, i.e. the movement goes beyond the intended point. This is called overshooting and is a manifestation of dysmetria.
  40. Thank You
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